1. Field of the Invention
The present invention relates to an ignition apparatus for internal combustion engine employing an electronic distribution system for supplying and shutting off a primary current to an ignition coil using a power transistor, and more specifically, to an ignition apparatus for internal combustion engine effectively preventing faulty operation caused when the supply of the primary current is started (when an ignition signal rises) without using a high-tension diode.
2. Description of the Related Art
Conventionally, an ignition apparatus for internal combustion engine employing an electronic distribution system having an ignition coil independently provided with each ignition plug controls an amount of fuel to be injected to each cylinder and ignition timing by electronic calculation using a microcomputer.
Although a primary current is supplied and shut off to the ignition coil by turning on and off a power transistor by an ignition signal at the time, there is a possibility that faulty operation such as early firing and the like may be caused by a high-tension secondary voltage induced when the ignition signal rises.
The conventional ignition apparatus for internal combustion engine has a high-tension diode inserted to the secondary side of an ignition coil to prevent the above faulty operation and prohibits the output of a high-tension secondary voltage when the ignition signal rises.
The conventional ignition apparatus for internal combustion engine will be described below with reference to FIG. 6 and FIG. 7. FIG. 6 is a circuit arrangement diagram showing the conventional ignition apparatus for internal combustion engine and FIG. 7 shows a waveform diagram explaining operation of the conventional apparatus shown in FIG. 6.
In FIG. 6, an ignition power unit 1 includes an ignition coil 13 composed of a primary coil 11 and a secondary coil 12 and a power transistor 14 for supplying and shutting off a primary current i1 to the primary coil 11 and applies a high-tension secondary voltage V2 which is output from the secondary coil 12 to the ignition plug of each cylinder (not shown).
A high-tension diode 15 for preventing faulty operation is inserted to the output terminal of the secondary coil 12 so that a positive polarity voltage to be superposed with the secondary voltage V2 can be removed. The primary coil 11 and secondary coil 12 in the ignition coil 13 has a common power distribution terminal connected to a battery power unit.
The power transistor 14 is composed of an emitter-grounded NPN transistor and has a collector connected to the primary coil 11.
A control circuit 2 includes a CPU 21 composed of a microcomputer and an output transistor 22 for amplifying a control signal from the CPU 21. The CPU 21 controls fuel injection to each cylinder of an internal combustion engine in accordance with operating state signals D from various sensors (not shown) as well as calculates ignition timing (corresponding to timing at which the primary current i1 is shut off) and a period for supply of the primary current i1 (corresponding to the pulse width T of an ignition signal G) and outputs the ignition signal G to the power transistor 14 through the output transistor 22.
The output transistor 22 is composed of an emitter-grounded NPN transistor having a collector connected to the battery power unit.
The ignition signal G is applied to the base of the power transistor 14 to supply and shut off the primary current i1 and causes the ignition coil 13 to produce a high-tension secondary voltage V2.
Note, the operating state signals D obtained from the various sensors include, for example, an engine r.p.m., amount of intake air, cooling water temperature, intake manifold pressure, degree of throttle opening, amount of accelerator depression and the like.
FIG. 7 shows waveform diagrams of various signals in FIG. 6 and shows the changes in time of the collector potential Vc (FIG. 7B) of the power transistor 14, primary current i1 (FIG. 7C) and secondary voltage V2 (FIG. 7D) each produced by the application of the ignition signal G (FIG. 7A).
Next, operation of the conventional ignition apparatus for internal combustion engine shown in FIG. 6 will be described with reference to FIG. 7.
First, the CPU 21 in the control circuit 2 injects fuel to each cylinder of the internal combustion engine at optimum timing in accordance with the operating state signals D as well as outputs the ignition signal G to optimize a period for supply of the primary current i1 and ignition timing (shut-off timing).
The power transistor 14 in the ignition power unit 1 is turned on in response to the ignition signal G at an H level and starts to supply the primary current i1 to the primary coil 11.
The ignition signal G is changed to an L level at optimum timing after the primary current i1 reaches a target current value and turns off the power transistor 14 to shut off the primary current i1. With this operation, the high-tension secondary voltage V2 is induced to the secondary coil 12 so that discharge spark is produced to each ignition plug to cause ignition.
When the collector potential Vc of the power transistor 14 steeply drops when the ignition signal G rises, however, an induction voltage is produced to the ignition coil 13 so that a relatively high-tension noise signal is superposed with the secondary voltage V2 as shown by a dotted line of FIG. 7.
If discharge spark is produced by the noise signal to the ignition plug of a cylinder in an intake process or compression process, ignition or firing is caused at undesired early timing.
To cope with this problem, the high-tension diode 15 is inserted to the output terminal of the ignition coil to output the secondary voltage V2 from which a superposed noise signal of positive polarity is removed.
With this arrangement, the effect of the secondary voltage V2 which is caused when the supply of the primary current i1 is started can be restrained to prevent faulty operation.
However, the insertion of the high-tension diode 15 results in the increase of the number of parts and circuit arrangements as well as the increase of the size and weight of the ignition apparatus because of the necessity for securing a parts mounting space and insulation space and further the increase of a job cost necessary to the assembly of the ignition coil 13, the connection to the secondary coil 12 and the like.
Further, since the high-tension diode 15 is not only applied with the high-tension secondary voltage V2 but also accommodated in the vicinity of the ignition coil 13 which generates high temperature, the high tension-diode 15 must have sufficient reliability to withstand adverse environment in which it is used, so that the cost of the diode is increased, by which the cost of the ignition apparatus as a whole is also increased.
As described above, since the conventional ignition apparatus for internal combustion engine includes the high-tension diode 15 inserted to the output terminal of the ignition coil 13 which produces the secondary voltage V2 to prevent faulty operation caused when the ignition signal G rises, there is a problem that the number of parts is increased to thereby increase the size of the ignition apparatus and the cost of the apparatus.
An object of the present invention made to solve the above problem is to provide an ignition apparatus for internal combustion engine capable of restraining faulty operation caused when an ignition signal rises without using a high-tension diode and reducing the size and cost of the apparatus.